In the field of filter elements that are tubular in shape, each membrane is constituted by a porous support made in the form of a tube whose inside surface is provided with at least one separating layer of nature and morphology that are adapted to separate molecules or particles contained in the liquid medium circulating inside the tube. By a sieve effect, such a membrane separates molecular or particular species from the substance to be treated insofar as all of the molecules or particles that are larger than the pore diameter of the membrane are stopped. The membrane thus subdivides the inlet volume into a first volume called the "filtrate" or the "permeate" and containing the molecules or particles that have passed through the membrane, and a second volume that contains the molecules or particles retained by the membrane.
Separation of molecular or particular species is mainly characterized firstly by stop size, i.e. the smallest molecule or particle that is totally stopped by the membrane, and secondly by the transit speed at which the separation is performed. The transit speed through the membrane depends on texture characteristics of the layer (pore diameter, porosity) and also on its thickness. To be effective, a membrane must have high transit speed and consequently small thickness. The function of the support is to provide mechanical strength enabling layers to be made that are very thin. The support also needs to possess the characteristic of very low hydraulic resistance so that the transit speed through its pores does not give rise to significant head loss, since that would reduce the efficiency of the membrane. Thus, the support must be capable of providing the membrane with mechanical strength while the separating layer must define permeability without participating in mechanical strength.
Numerous membranes made on the basis of tubular filter elements are known in the state of the art. Thus, it is known to mount tubular elements in parallel inside a case referred to as "module". In conventional manner, modules are characterized by the ratio of the total exchange area of the membranes divided by the volume of the module. Naturally, the idea is to provide modules for which the value of this ratio is as high as possible. To increase the ratio, it is clear that it is appropriate to reduce the outside and inside diameters of the tubes. However, it should be considered that inorganic membranes possess the characteristic of being fragile, such that it appears to be impossible in an industrial context to make use of small diameter tubes.
To increase the ratio while avoiding the problem of fragility, a novel type filter element has been developed that has multiple channels. As can be seen in FIG. 1, this inorganic filter element 1 comprises a rigid porous support 2 of elongate shape having a right cross-section that is polygonal or, as illustrated, circular. The porous support 2 which is made of a ceramic, for example, is formed to include a series of channels 3 parallel to the longitudinal central axis A of the support 2. The surface of each channel 3 is covered in at least one separation layer that is intended to come into contact with the medium to be separated. The exchange area of a multichannel filter element is much greater than that of a tube, thereby making it possible to increase the ratio of exchange area over volume.
However, it turns out that the multichannel element suffers from a major drawback. During separation, fluid transfer takes place through the layer(s) and then the fluid spreads out in the pores of the support so as to reach the outside surface 4 of the support. As can be seen clearly in FIG. 1, the path that the filtrate must follow before reaching the outside surface 4 of the support is much longer for the channel(s) situated in the central portion of the support than it is for the other channels, in particular the peripheral channels. Furthermore, filtrates coming from the channels in the central region of the support encounter the filtrates coming from the other channels. That is why head loss appears for transfer of filtrate to the outside surface of the support. This head loss opposes transfer pressure and reduces transit speed. It should also be observed that head loss exists, to a lesser degree, for the other channels. Because of the observed head loss, the transit speed of a multichannel element having n channels is less than that which would be provided by a collection of n tubular elements having the same inside diameter as the channels, and that reduces the advantage of increasing the ratio of exchange area over volume.
In the state of the art, another kind of multichannel type tubular filter element is known from patent application WO 93/07 959. In a first embodiment, the filter element has an inorganic porous support in which channels are provided parallel to the central axis of the support with their centers being situated on a circle that is coaxial about the central axis. In right cross-section, each channel has a peripheral wall directed towards the outside surface and co-operating therewith to define a passage of constant thickness through which the filtrate passes. Each peripheral wall is extended on either side by interconnected radial walls each defining a partition in co-operation with the facing radial wall of an adjacent channel. The channel profile is selected so as to leave partitions that flare in a wedge shape towards the outside of the substrate. In a second embodiment, the axes of the channels are situated either on a plurality of coaxial circles about the axis of the support, or else in layers that are parallel to one another and to the axis of the porous support. In that second embodiment, adjacent channels between the two series leave a partition that also flares towards the outside of the substrate. It thus appears that the wedge shape facilitates transfer of the permeate towards the outside surface of the support.
Although the second embodiment facilitates transfer of the permeate, it must be observed that that configuration does not provide a solution to the problem of loss of permeability.
However, the Applicant has shown that the first configuration does make it possible to reduce head loss for permeate transfer, so that permeability can be obtained that is very close to that of tubular elements having the same inside diameter as the channels. In that disposition, the passages along which the filtrate passes are of constant thickness and carry fluid coming solely from the corresponding peripheral walls of the channels. The transfer of filtrate coming from the radial walls takes place via the pores of the partitions, and the constantly varying thickness of said partitions enables the filtrate to be evacuated without head losses appearing to oppose the transfer pressure.
Nevertheless, this first type of configuration for a filter element suffers from drawbacks.
The partitions that remain between the channels are of increasing thickness such that the ratio between the thickest portion and the thinnest portion is no more than three. It turns out that this variation in partition thickness prevents layers being deposited that are uniform. The available pore volume increases continuously, with the consequence that the thickness of a deposit increases going from the thinnest portion towards the thickest portion. Although this matters little for microfiltration layers, such variation in thickness gives rise to faults due to excess thickness in ultrafiltration layers and is completely unacceptable for nanofiltration layers.
Also, in a variant embodiment, the text of that document provides for the variation in partition thickness to appear beyond one-half the total height of the partition. That configuration is even worse for uniformity of deposits, since it gives rise to a sudden change therein. This gives rise to cracks appearing because of tension during the shrinkage that is caused by drying.
Another drawback is associated with the fact that said prior solution provides for connection fillets between the radial and peripheral walls, said fillets presenting sharp angles. It has been observed, that under such conditions meniscuses form in the connection zones while the deposition solution is being emptied out. Such liquid meniscuses give rise to localized accumulation of the deposited suspension, and that is a major source of defects because of the large variation in deposit thickness that is caused thereby.